The present invention relates to carbon bodies whose surfaces are covered with a layer of silicon-infiltrated silicon carbide (=SiSiC) and to a process for producing them starting from carbon components.
Graphite pebbles are used, inter alia, in nuclear-power engineering. In this connection it is troublesome that, as a result of the low hardness of the graphite, finely divided abraded material is produced in large amounts if the pebbles are subjected to mechanical shear forces. Furthermore, graphite components are used in electric furnaces in the presence of liquid silicon in the production of SiSiC bodies. These include, for example, combustion aids, heating elements and structural components in the graphite heating zone. In this case, it is a disadvantage that graphite in direct contact with molten elemental silicon reacts to form silicon carbide. The liquid silicon passes through the relatively large pore system of the graphite which is, for example, protruded, into the carbon body and is exothermically converted into silicon carbide. The larger the pore system and the particle size are, the better liquid silicon is able to post-diffuse. Since the silicon carbide formed takes up more space than the reacting carbon because of its crystal structure, this results in the formation of cracks and splits on the component. Disadvantageously, this mechanism leads to complete destruction of the component within a very short time.
However, graphite bodies which are produced by an iso-static pressing process and have a comparatively high density, a fine-grained structure and low porosity behave differently. These graphite bodies react in principle in the same way towards liquid silicon; after a first reaction at the surface of the graphite to form silicon carbide, the narrow pore channels, however, suppress a further post-diffusion of silicon into the interior of the component since the increase in volume accompanying the reaction reduces the pore system or seals it completely. Such graphite grades are, however, not available in all component dimensions and, in addition, are substantially more expensive.